1. Field of the Invention
The present invention relates to a servo information recording method and a magnetic storage apparatus.
2. Description of the Related Art
Applications of the magnetic storage apparatus such as an HDD (Hard Disk Drive) are increasing, and importance of the magnetic storage apparatus is increasing. In addition, in the magnetic recording medium such as a magnetic disk, the recording density is increasing at a rate of over 50% per year, and this trend may be expected to continue in the future. With this trend of increasing recording density, magnetic heads and magnetic recording media suited for the high recording density are being developed.
The magnetic storage apparatus may be provided with the so-called perpendicular magnetic recording medium in which an axis of easy magnetization within a recording layer is mainly oriented in a perpendicular direction. In the perpendicular magnetic recording medium, the effects of demagnetization is small in a boundary region between recorded bits, and a sharp bit boundary may be formed, to thereby suppress increase of noise, even when the recording density is high. In addition, in the perpendicular magnetic recording medium, the decrease in the recording bit volume caused by the high recording density is small, and the thermal stability of written bits (or characteristic to withstand heat fluctuation) may be improved. The thermal stability of written bits may also be simply referred to as “thermal stability”.
In order to cope with the demands to further improve the high recording density of the magnetic recording medium, studies are being made to use a single-pole head having a desirable write performance with respect to the perpendicular recording layer. More particularly, a proposed magnetic recording medium is provided with a back layer made of a soft magnetic material between the perpendicular recording layer and a nonmagnetic substrate, in order to improve the input and output efficiency of magnetic flux between the single-pole magnetic head and the magnetic recording medium.
In the conventional magnetic disk, a servo information region recorded with servo information, and a data region to which data are recorded and from which the data are reproduced, are provided on the same plane, that is, in the same recording layer. On the other hand, Japanese Laid-Open Patent Publication No. 2003-228801 proposes a system that forms the recording layer of the magnetic disk from a lower layer part and an upper layer part having a coercivity lower than that of the lower layer part, and records the servo information in the lower layer part having the higher coercivity and records the data in the upper layer part having the lower coercivity. According to this proposed system, the servo information region recorded with the servo information, and the data region to which data are recorded and from which the data are reproduced, may be provided in separate recording layers. In this case, the servo information region and the data region may overlap in a plan view of the magnetic disk, and the data region may be increased compared to a case in which the servo information region and the data region are provided in the same recording layer.
On the other hand, in the magnetic disk used in the HDD that is an example of the magnetic storage apparatus, the recording of the servo information is performed only once in many cases. This recording is performed by a dedicated STW (Servo Track Writer) used by a manufacturer of the HDD. However, according to the conventional recording system that uses the STW, it is difficult to increase the track density of the magnetic disk that is to be recorded with the servo information in the lower layer part having the coercivity than the upper layer part in the recording layer, for the following reasons. That is, because a distance between the magnetic head and the lower layer part is relatively long, the intensity of the magnetic field output from the magnetic head and reaching the lower layer part becomes low, and the recording characteristic of the magnetic head with respect to the lower layer part deteriorates. In other words, because the coercivity of the lower layer part of the recording layer is high compared to that of the upper layer part of the recording layer, the deterioration of the recording characteristic of the magnetic head becomes notable.
In order to improve the recording characteristic of the magnetic head, it is necessary to increase the intensity of the magnetic field output from the magnetic head, however, there is a limit to increasing the intensity of the magnetic field that may be output from the magnetic head. In addition, when a width of a recording element of the magnetic head is made wide in order to increase the intensity of the magnetic field, it becomes difficult to record on a narrow track, and the track density deteriorates to thereby deteriorate the recording density of the data region.
Furthermore, when the distance between the magnetic head and the lower layer part of the recording layer increases, the magnetic field from the magnetic head spreads to the outside and makes the recording on the narrow track difficult. In this case, the track density also deteriorates to thereby deteriorate the recording density of the data region.
Therefore, according to the magnetic storage apparatus, such as the HOD, it is becoming difficult to improve the track density in the data region of the magnetic recording medium in order to improve the recording capacity.